This invention relates to cementitious compositions and to accelerating admixtures for use therein, especially as shotcrete accelerators.
Sprayed concrete or xe2x80x9cshotcretexe2x80x9d, which is applied to substrates such as rock faces and tunnels by spraying must set very rapidly. In such a use, the traditional concrete accelerators such as calcium chloride are not effective, and more powerful accelerators must be used, including sodium aluminate and alkali metal hydroxides. These are highly alkaline, which gives rise both to handling difficulties and unpleasant conditions when spraying in confined spaces such as tunnels.
An alternative accelerating technology based on aluminium compounds has been known for some time. The use of aluminium hydroxide either alone or combined with other materials is described in, for example, Japanese Kokai S51-144425 and European Patent 0 076 927.
In a variation on this, aluminium hydroxide may be partially reacted with an acid, preferably an organic acidxe2x80x94see, for example GDR Patent DD 226 344, British Patent GB2 307 475 and European Application 0 798 300.
Aluminium sulphate is also known as a component of accelerating systems, and is described in, for example, Japanese Kokais S50-143816, S58-99147 and S63-291839, and European Patent Application 0 812 812. Another aluminium compound which has been used in concrete accelerators is basic aluminium sulphate (also known as aluminium hydroxysulphate)xe2x80x94see, for example, European Patents 0 711 260 and 0 775 097.
It has now been found that a particularly effective accelerating admixture can be prepared by a combination of particular components. There is therefore provided, according to the present invention, an accelerating admixture for sprayed concrete, comprising
(a) the reaction product of aluminium hydroxide with an organic acid;
(b) aluminium sulphate; and
(c) at least one alkanolamine.
Reaction products of aluminium hydroxide and organic acids are known to the art, and are described in, for example, the abovementioned DD 226 344 and GB 2 307 475. The products have the formula Al(OH)3xe2x88x92xRx, where R is an anion of an organic acid. The acid is preferably a carboxylic acid, more preferably a lower (C1-C6) alkanoic acid and most preferably either formic acid or acetic acid, most preferably formic acid. The value of x can vary across a wide range, as is disclosed in these patents, and the same values apply in this invention.
Aluminium sulphate is a well-known and readily-available raw material and any of the commercial grades are suitable for use in this invention, for example the so-called xe2x80x9c12%xe2x80x9d and xe2x80x9c17%xe2x80x9d grades (referring to the theoretical weight percentage of aluminium oxide present).
The alkanolamine for use in this invention may be selected from any such material known to the art. The preferred alkanolamines are diethanolamine, triethanolamine and methyldiethanolamine. A mixture of two or more such materials may be used.
The weight proportions of the components which are combined to form the accelerating admixtures according to the invention are
the remainder to 100% being water. The preferred formic acid is quoted here, but the same ranges apply to the other C1-C6 alkanoic acids.
With regard to the alkanolamines, the xe2x80x9cwidest rangexe2x80x9d given corresponds to the extremes of the different alkanolamines. The proportions of the various alkanolamines vary substantially. For example, the actual xe2x80x9cwidest rangesxe2x80x9d for the three preferred materials are
However, the skilled person can readily ascertain what works and the xe2x80x9cpreferred rangexe2x80x9d is valid for all alkanolamines.
In an especially preferred embodiment of the invention, the admixture may additionally contain ferric sulphate, to the extent of from 0. 1-1.0% by weight of cement. The addition of this material produces a noticeable improvement in performance.
The admixture according to the invention may be manufactured by any convenient means. The order of addition of components is not critical. Thus, for example, it is possible and permissible first to react the formic acid and the aluminium hydroxide to form products of the type described in the abovementioned DD 226 344, and then to add the other components thereto. The reaction is carried out at elevated temperature, no higher than 90xc2x0-95xc2x0 C. Alternatively, the aluminium hydroxide may be added last, to a heated mixture of the other components. This is a preferred method.
The admixture of the present invention may be used in an art-recognised manner with shotcrete. The invention therefore provides a process of coating a substrate with concrete by preparing a sprayable concrete mix and spraying this mix on to the substrate through a nozzle, there being supplied to the mix at the nozzle an accelerating admixture as hereinabove described. The admixture is provided to the nozzle as an aqueous solution or suspension at typically 50-60% solids by weight of the suspension, and so that the concrete receives from 0.5 to 15% by weight of admixture (solids on cement). The actual figure used will vary, depending on the type of cement and the mix design, but the provision of a suitable quantity is well within the skill of the art.
The invention therefore additionally provides a method of applying a layer of concrete to a substrate by spraying it through a nozzle, wherein there is added at the nozzle an accelerating amount of an accelerating admixture as hereinabove described.
The admixtures of the invention bring about unusually fast setting in comparison with other known alkali-free accelerators.
The invention further provides a layer of hardened concrete applied by spraying to a substrate via a nozzle, hardening having been accelerated by the addition at the nozzle of an accelerating amount of an accelerating admixture as hereinabove described.